A variety of assembly processes are becoming more and more automated. In the fabrication of printed circuit boards with surface mount components, for example, printed circuit boards can pass from a station that automatically applies a pattern of solder paste to the unloaded board to one that mechanism to register the workpiece (the printed circuit board at hand) in the machine. In the past this has included the use of dowel pins and fences. Owing to their mechanical nature, such methods are not by themselves particularly flexible. Attempts to adapt an existing system of that sort to process parts that are significantly different in size than those originally envisaged can prove difficult.
In the application sketched above, as well as in many other similar ones, it can be advantageous to carry one or more individual workpieces upon a carrier or frame. The workpieces are held firmly in place upon the carrier, but not necessarily precisely in any particular position. Each carrier is separately identified and recognizable (e.g., with machine readable bar coding). At or before the first workstation the particular relative location(s) upon the carrier of the one or more workpieces is determined by a precision locating mechanism. This operation may involve automatic sensing and pattern recognition, and may require the use of targets affixed to or built into the workpiece. The locations of the workpieces are determined relative to some indexing or registration mechanism that positions the carrier at each workstation where an operation is to be performed upon they workpiece. For the sake of simplicity, assume that the same indexing or registration scheme is used for all carriers, and that all carriers are alike. A programmable control mechanism (e.g., a computer) stores the particular relative locations arising out of this particular loading of that carrier with a particular collection of workpieces, and associates those with that particular carrier. Thus, the workpiece locating operation is done only once each time a carrier is loaded with a workpiece. As the carrier proceeds from workstation to workstation each determines (or is perhaps told) which carrier is at hand. After registering the carrier the workstation can then, without further measurement or investigation, position or align either its operating head or the carrier according to the offsets (i.e., the relative location) for the particular workpiece to be operated upon. A further advantage of the invention is that once a carrier and its workpiece have been characterized, they need not necessarily proceed through the sequence of workstations next in order after the carrier/workpiece combination that was just previously characterized. That is, the order of the carriers can be arbitrary.
As an extension of the technique described above, it may be desirable to allow a variable number of workpieces to occupy the carrier. The actual number for each carrier would than be determined automatically at the same time as are the relative locations. Then as the carrier goes from workstation to workstation each will do the appropriate number of operations. And if a workstation determines during its operation that a workpiece is fatally defective, it can "remove" that workpiece from the carrier by deleting it from the list describing the work in progress. Then the next workstation would proceed with its operation as if the carrier had been loaded without that particular workpiece to begin with.